Kevin Aylward B.Sc.
Power - Accuracy - Frequency Limit (PAFL) Of Bipolar Amplifiers
Back to Contents
This paper addresses the theoretical limit of any bipolar amplifier with regards to the constraints of Power (current), Accuracy, and Frequency. Suitable modifications to the theory are easily made for cmos amplifies, and are addressed in another paper.
Analogue design is always one of performance tradeoffs. One can rest assured that any tweak of a design that improves one aspect of its performance will always result in another aspect of its performance being inferior. It would therefore seem, that it should be possible to identify some key performance features, and construct some design equations that expressed such design limitations. This paper shows that for a given bipolar process, there is an inherent limit that links together, power (current), accuracy (untrimmed) and speed (gain bandwidth), thereby making it much clearer as to how to more optimally effect such design compromises in practice.
The initial motivation for this paper was based largely on the realization that principles behind the Heisenburg uncertainty relation regarding time and energy might be applied to electronic design. Clearly, it was not anticipated that the quantum mechanical limit itself would impact a typical analogue design, and that is indeed the case. However, its analysis certainly highlights some of the fundamental issues involved.
As a starter then, the Heisenburg time-energy relation is given by:
where ΔE is the rms uncertainty in energy and Δt is the time that the system ( energy) remains in a given state. Given that the energy state is changing with time, this energy must be measured within this time period.
With a little hand waving (appendix C), the following relation can be derived:
The details of this result are not overly important, the key point being that this indicates that there is a minimum power for a signal or device, subject to a constraint of accuracy and speed of operation. Plugging in some numbers gives us nW's of power for GHz, 20bit signals, so the quantum limit is obviously no practical limit in terms of real designs. However, it does illustrate the basic connection between power, speed and accuracy. That is, there is always a relation of the form:
PAFL Power - Accuracy - Frequency Limit
It is first noted, that this derivation relies on suitable approximations that are usually reasonable valid in most situations. The purpose of the derivation is to formulate a reasonable "best case" such that the real circuit will generally always be worst. This allows an immediate determination of whether a given specification is achievable from the outset.
In appendix A, it is shown that, given an emitter area of a bipolar transistor, a relation can be formed relating this area of a bipolar transistor to how accurately it matches another bipolar transistor. This relation is:
where σ is the relative error and α is a constant.
This should be quite an intuitive equation. It simply says that the matching error is inversely proportional to the square root of the total area of the bipolar transistor. Typically, manufactures characterize their process and empirically determine the value for alpha. It can therefore be taken as a known constant, just as any other process characteristic.
The starting point for this is the well-known capacitor current equation:
This equation represents an output voltage swinging across a capacitor load. If one considers a bipolar transistor single transistor amplifier driven by a pure voltage source, the main capacitance across the transistors output is the base collector capacitance and the collector substrate capacitance. The base emitter capacitance, including the diffusion capacitance, will usually have negligible effect, assuming low base resistance, with this type of voltage drive. The collector emitter capacitance can usually be ignored as well. It might be argued that a cascode connection might be faster, however, it should be noted that cascodes generally only offer an advantage when there is significant resistance in the base drive that results in a significant Miller roll off.
It is now noted that bipolar transistor capacitance's are proportional to the total area of the device, so that C in (2) above can be expressed as:
where Ccbf, Ccsf are the respective specific capacitance's of the base and collector substrate per unit area respectively.
Substituting (3) into (2)
Squaring (1) and Substituting into (4) for A
In appendix B it is shown that the small signal output current from a bipolar transistor amplifier, when biased with a current I, is given by:
where Vt = KT/q and Vi is the small signal input voltage, and where K is boltzman's constant, q is the electronic charge and T is the absolute temperature of the device.
Since this current is the current that charges the load current, equations (5) and (6) can be equated to each other:
Aylward's PAFLimit for bipolar transistor amplifies.
Thus, it can be seen that the minimum operating current of a bipolar transistor amplifier depends on the product of the amplifier's gain-bandwidth and square of the accuracy, along with the process capacitances.
Some values are presented here in order to gain some insight into the limit of a typical process.
Typical error σ = 10% = 0.1 at A=16u2, so that α =0.4u
Thus M is:
From which the following table can be constructed:
Desired Desired Required
GBW σ I
1GHz 0.1 5.02ua
100MHz 0.1 0.5ua
10Mhz 0.1 -
1GHz 0.01 502ua
100MHz 0.01 50.2ua
10Mhz 0.01 5.02ua
It should be noted that the above is a raw, best case calculation. Other considerations may prohibit the higher currents from being realized in practice.
It has been shown that there is an inherent power, accuracy and gain-bandwidth limitation for any specific bipolar transistor fabrication process. It is anticipated that the relations derived in this paper may result in more optimal design of bipolar amplifiers.
Derivation of :
It is well known that a reasonable accurate large signal design equation for bipolar transistors in the saturation region is given by:
This appendix does the hand waving argument on the quantum limit of power accuracy and frequency from:
The uncertainty in energy can be expressed by:
Where E is the energy being measured, or equivalently the energy being produced. Note that
This energy can be related to the power via:
This paper may be reproduced so long as no
charge is made http://www.kevinaylward.co.uk Last updated 29th October 20 Copyright © Kevin Aylward 1997-2009. KANA-EE-08/01/02-BIPOLARPAFL0001
and that the paper is reproduced in full with full credit given to it's author.
Last updated 29th October 2009
Copyright © Kevin Aylward 1997-2009.